This invention relates to peroxide-curable fluoropolymer compositions having interpolymerized units of a C2-C10 fluorinated olefin, which may contain one or more heteroatom(s), and a hydrogen-containing C2-C9 olefin, which may be made in the presence of a brominated salt. The invention also relates to methods of making such fluoropolymer compositions and articles made from the cured compositions.
Fluoroelastomers having up to one iodine or bromine atom at the end of a polymeric chain, with a bromine content in the chain from 0.05-2 weight percent (wt %) are described in U.S. Pat. No. 5,173,553. These fluoroelastomers are said to have improved proccessability, particularly in injection molding processes, mold release, and heat stability. The fluoroelastomers are made from fluorinated monomeric units of vinylidene fluoride and/or tetrafluoroethylene, and optionally hexafluoropropene and/or perfluoroalkyl-perfluorovinylethers. These fluorinated copolymers can be combined with amounts up to 40 mole percent (mol %) of monomeric units derived from ethylene, propylene, butene, and/or isobutylene.
Copolymers of tetrafluoroethylene and propylene have been prepared and cured with peroxides. The physical properties of such polymers after curing have undesirably high compression set values.
Briefly, the present invention provides a curable composition comprising (a) a fluoropolymer comprising interpolymerized units derived from (i) CF2xe2x95x90CFxe2x80x94Rf, wherein Rf is fluorine or a C1-C8 perfluoroalkyl, (ii) at least about 10 mole percent of a hydrogen-containing C2-C9 olefin, based on the total moles of said CF2xe2x95x90CFRf and said olefin, (iii) an average of two or more bromine atoms including one or more terminal bromine atom(s) per fluoropolymer chain, (iv) optionally CX2xe2x95x90CXxe2x80x94R, wherein each X is independently H, F, or Cl and R is a halogen or a C1-C8 alkyl or alkenyl group that may include one or more ether linkage(s), (v) optionally a bromine-containing cure site monomer; (b) optionally, a peroxide curing agent; and (c) optionally, a crosslinking co-agent. The fluoropolymer in the curable composition preferably is polymerized in the presence of a bromine salt.
In another aspect, the invention provides a cured fluoroelastomer composition comprising the reaction product of (a) a copolymer comprising tetrafluoroethylene and propylene having an average of two or more bromine atoms including one or more terminal bromine atom(s) per polymer chain, (b) a peroxide curing agent, and (c) optionally, a crosslinking co-agent; wherein the composition has a compression set value lower than a comparative copolymer having a similar comonomer ratio and having an average of less than two bromine atoms per chain. This comparative copolymer is similar to the inventive fluoropolymer in that the ratio of monomers are within about 5 mol % or even closer, but the comparative material has lower amounts of bromine such that it contains less than two bromine atoms per polymer chain and/or has fewer than the one or more terminal bromine atoms of the inventive fluoropolymer.
In still another aspect, the present invention provides a method of preparing a fluoropolymer composition comprising (a) combining CH2xe2x95x90CFxe2x80x94Rf, wherein Rf is fluorine or a C1-C8 perfluoroalkyl, with at least about 10 mole percent of a hydrogen-containing C2-C9 olefin, based on the total moles of said CH2xe2x95x90CFRf and said olefin, a brominated salt, optionally CX2xe2x95x90CXxe2x80x94R, wherein each X is independently H, F, or Cl and R is a halogen or a C1-C8 alkyl or alkenyl group that may include one or more ether linkage(s), and optionally a bromine-containing cure site monomer, together with an initiator, and polymerizing the combination.
In still another aspect, the present invention provides fluoropolymer articles containing cured compositions.
The invention provides fluorinated elastomers (e.g., tetrafluoroethylene-propylene or xe2x80x9cTFE-Pxe2x80x9d) that are made in the presence of radical initiators and a bromine salt. Surprisingly, the initiator system of the present invention provides curable fluoropolymer, such as TFE-P, with excellent physical properties even where cure site monomers are absent from the fluoropolymer. These fluoroelastomers exhibit improvements in physical properties. Another advantage of the present invention is that the fluoroelastomers of the invention, after curing, exhibit significantly reduced compression set as compared to known TFE-P elastomers.
Other features and advantages of the invention will be apparent from the following detailed description of the invention and the claims. The above summary of principles of the disclosure is not intended to describe each illustrated embodiment or every implementation of the present disclosure. The following detailed description more particularly exemplifies certain preferred embodiments utilizing the principles disclosed herein.
The fluoroelastomer compositions of the invention are derived from interpolymerized units of fluorinated monomers, having the formula CH2xe2x95x90CFxe2x80x94Rf, wherein Rf is fluorine or a C1-C8 perfluoroalkyl, along with hydrogen-containing C2-C9 olefins, which have less than half of the hydrogen atoms substituted with fluorine, more preferably less than one-fourth of the hydrogen atoms substituted with fluorine, and which are non-fluorinated in other embodiments.
The polymer may optionally contain one or more bromine-containing cure site monomer such as bromotrifluoroethylene (BTFE), bromodifluoroethylene (BDFE), bromotetrafluorobutene (BTFB), perfluoro vinyl ethers having one or two fluorine atoms substituted with bromine atoms, an iodine-containing cure site monomer, and the like. The amount of bromine-containing cure site monomer in the fluoropolymer is up to about 5 mole percent (mol %) or less. The amount of bromine-containing cure site monomer, when included, preferably is from about 0.01 to about 4 mol %, and more preferably from about 0.05 to about 2 mol %, based on the total fluoropolymer composition. In cases where the fluoropolymer contains a bromine-containing cure site monomer, the amount of the hydrogen-containing C2-C9 olefin is at least 10 mol % of the total fluoropolymer composition. Propylene is one preferred C2-C9 olefin.
The curable compositions have an average of two or more bromine atoms per fluoropolymer chain. These bromine atoms typically are found at the polymer chain ends, i.e., terminal bromine atoms. Bromine atoms also may be found substituted along the polymer backbone, such as when a linear polymer is polymerized using, e.g., a brominated perfluorovinyl ether or a bromotetrafluorobutene (BTFB) cure site monomer. These fluoropolymers have at least one terminal bromine atom.
In another embodiment of the invention, a bromine-containing cure site monomer is not necessary to enable crosslinking of the inventive fluoroelastomer compositions. In this aspect, the curable compositions have an average of two or more terminal bromine atoms per fluoropolymer chain. Such terminal bromine atoms occur on the main chain and/or chain branches.
The fluoropolymer chains in the inventive composition may be linear, branched, or a combination thereof. The curable compositions yield fluoroelastomer with excellent, physical properties, including low compression set, whether or not a bromine-containing cure site monomer is included. In contrast, comparable materials similar in every respect except having less than one bromine atom per chain do not cure to provide desirable properties. Also in contrast, comparable materials having levels of bromine-containing cure site monomers similar to the inventive fluoropolymer but lacking the additional bromine atoms at the polymer chain ends (provided in the inventive fluoropolymer) have much higher compression set levels.
When the fluoropolymer does not contain a cure site monomer, the amounts of the hydrogen-containing C2-C9 olefins ranges from at least about 10 mol %, more preferably at least about 15 or even 20 mol %. In other embodiments, the hydrogen-containing C2-C9 olefin content ranges from at least about 25 mol %, more preferably at least about 30 or even at least about 40 mol %. The amounts of these hydrogen-containing olefins ranges from below about 90 mol %, more preferably below about 80 mol %. In some preferred embodiments, these fluoropolymers contain vinylidene fluoride in amounts from about 0.5 to about 20 mol %.
Olefins useful in the invention include those of the formula:
CX2xe2x95x90CXxe2x80x94R,
wherein each X is, independently, hydrogen or fluorine or chlorine, R is hydrogen, fluorine, or a C1-C12, preferably C1-C3, alkyl. Preferred olefins include partially-fluorinated monomers (e.g., vinylidene fluoride) or hydrogen-containing monomers such as olefins including xcex1-olefins (e.g., ethylene, propylene, butene, pentene, hexene, and the like). Combinations of the above-mentioned materials are also useful.
Perfluorinated vinyl ethers also are suitable as comonomers in the present invention. Such perfluorovinylethers include, for example, CF2xe2x95x90CFOCF3, CF2xe2x95x90CFOCF2CF2OCF3, CF2xe2x95x90CFOCF2CF2CF2OCF3, CF2xe2x95x90CFOCF2CF2CF3, CF2xe2x95x90CFOCF2CF(CF3)OCF2CF2CF3, and CF2xe2x95x90CFOCF2CF(CF3)OCF2CF(CF3)OCF2CF2CF3.
The fluoroelastomer compositions of the present invention are prepared by any known method. One inventive method is a free radical polymerization process, carried out in an aqueous emulsion, in the presence of a bromine-containing salt. Bromine-containing salts include compounds capable or providing bromine ions in the polymerization medium and conditions. These include, for example, KBr, NH4Br, other salts of HBr, bromides of the metals belonging to groups I and II, A and B of the Periodic System, such as for example Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Cu, Ag, Zn, Cd, as well as the transition metals, such as for example Fe, Co, Ni, Ru, Rh, Pd, Pt, or belonging to groups Ill and IV B of the Periodic System, such as for example Al, Ga, Sn, Pb. The bromides or the alkaline or alkaline-earth metals arc preferably used. The amount of such compounds capable of providing bromide ions, to be used in the process of the invention, is that amount effective to produce the desired results, such as the desired level of compression set resistance. The molar ratio of bromide salt compared to the initiator is preferably between about 1:0.1 and 1:10, more preferably bctwccn about 1:0.5 and 1:5. In addition to the above-mentioned bromine-containing salts, alkyl ammonium salts, such as NR4+Br, and HN(R)3+Br, wherein R is a C1-C9 alkyl can be used. Still another useful class of bromine-containing compounds is the imides, such as N-Bromosuccinimide. Preferred bromine salts include KBr, FeBr2, CuBr, and quaternary salts such as NH4Br.
The curable fluoropolymer of the invention comprises an average of two or more bromine atoms (of which one or more bromine atom(s) are terminal) per fluoropolymer chain. When less than one bromine atom per fluoropolymer chain is used, the quantity of these atoms is too low to crosslink the fluoroelastomers into a network having desirable physical properties such as low compression set.
Optionally during a free radical polymerization process, bromine-containing chain transfer agents (CTA) can be used, such as a brominated compound that contains one or two bromine atoms and is sufficiently stable that undesirable side reactions do not proceed under the desired polymerization conditions.
In some embodiments of the present invention, no chain transfer agents (CTA) are used, as such materials result in undesirable organic residues and reduce the rate of polymerization. In such an embodiment, the resulting fluoropolymer is substantially free of residue from a chain transfer agent. That is, the fluoropolymer contains less than about 0.1 wt %, preferably less than 0.05 wt %, more preferably 0 wt % residue from a CTA. Such chain transfer agents are known in the art, e.g., U.S. Pat. No. 4,000,356 to Weisgerber, et al., which is herein incorporated by reference.
The preferred APS/KBr system of the present invention provides one or more advantages including improving the polymerization rate, leaving no undesirable organic residue, increasing the efficiency of the polymerization, and providing fluoropolymers having low compression set features. Such polymers may contain residue from an inorganic bromine salt, or this residue can be removed via known methods.
The inventive compositions can be elastomers or thermoplastics.
One example of a useful fluoropolymer consists essentially of principal monomer units of tetrafluoroethylene and at least one olefin. In this embodiment, the copolymerized olefin units comprise from at least about 10 to about 70 mol % (more preferably 35 to 60 mol %) of total monomer units present in the copolymer.
Surprisingly, the polymers made with the bromine-containing materials have much better properties, such as lower compression set, than comparative polymers made via processes lacking these materials.
The fluoroelastomer compositions of the present invention surprisingly can be cured using one or more known curing agents, preferably peroxide compounds, optionally along with one or more crosslinking coagents, even in the absence of a cure site monomer. Suitable peroxide curatives generally are those which generate free radicals at curing temperatures, such as those described in the art, e.g., WO 99/48939, the disclosure of which is herein incorporated by reference. Dialkyl peroxide and bis(dialkyl peroxide), each of which decomposes at a temperature above 50xc2x0 C., are particularly preferred, wherein the alkyl group has from 1 to 12 carbon atoms (more preferably 1 to 6 carbon atoms) and may be linear or branched. In many cases it is preferred to use a di-tertiarybutyl peroxide having a tertiary carbon atom attached to peroxy oxygen atom. Among the preferred peroxides of this type are 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hex-3-yne and 2,5-dimethyl-2,5-di(tertiarybutylperoxy)-hexane. Additional examples of useful peroxides include compounds such as dicumyl peroxide, dibenzoyl peroxide, tertiarybutyl perbenzoate, a,axe2x80x2-bis(i-butylperoxy-diisopropylbenzene), and di[1,3-dimethyl-3-(t-butylperoxy)-butyl]carbonate. Generally, about 1 to 3 parts of peroxide per 100 parts of perfluoroelastomer is used.
The fluoroelastomer compositions can include any of the adjuvants commonly employed in curable fluoroelastomer formulations. Optionally, one or more crosslinking co-agents may be combined with the peroxide curative. For example, one material often blended with a fluoroelastomer composition as a part of a curative system is a coagent (sometimes also referred to as a co-curative) composed of a polyunsaturated compound that is capable of cooperating with the peroxide curative to provide a useful cure. These coagents are particularly useful in combination with a peroxide curative. The coagent(s) can generally be added in an amount equal to between 0.1 and 10 parts coagent per hundred parts fluoropolymer (phr), preferably between 1 and 5 phr. Examples of coagents useful with the peroxide curative compound of the present invention include triallyl cyanurate; triallyl isocyanurate; tri(methylallyl) isocyanurate; tris(diallylamine)-s-triazine; triallyl phosphite; N,N-diallyl acrylamide; hexaallyl phosphoramide; N,N,Nxe2x80x2,Nxe2x80x2-tetraalkyl tetraphthalamide; N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl malonamide; trivinyl isocyanurate; 1,3,5-trivinyl-trimethylcyclotrisiloxane; and tri(5-norbornene-2-methylene)cyanurate. Particularly useful is triallyl isocyanurate. Other useful coagents include the bis-olefins disclosed in EP 0 661 304 A1, EP 0 784 064, A1 EP 0 769 521 A1, and U.S. Pat. No. 5,585,449.
Fluoropolyrners that do not contain a cure site monomer can be cured by either using the above described peroxide cure systems with a coagent or through known crosslinking reactions such as those using bisphenols/onium-systems. Mixed cure systems (peroxides/bisphenols) also can be used. In these cases, the polymers often have a certain amount of VDF incorporated.
Additives such as carbon black, stabilizers, plasticizers, lubricants, fillers, and processing aids typically utilized in fluoroelastomer compounding can be incorporated into the compositions, provided that they have adequate stability for the intended service conditions.
One or more known acid acceptors can also be added to the inventive composition. However, where the presence of extractable metallic compounds is undesirable (such as for semiconductor applications) the use of inorganic acid acceptors should be minimized, and preferably avoided altogether. Useful acid acceptors include, for example, zinc oxide, calcium hydroxide, calcium carbonate, magnesium oxide, etc.
The curable fluoroelastomer compositions can be prepared by mixing one or more fluoroelastomer(s), the peroxide curative, any selected additive or additives, any additional curatives (if desired), and any other adjuvants (if desired) in conventional rubber processing equipment. The desired amounts of compounding ingredients and other conventional adjuvants or ingredients can be added to the unvulcanized fluorocarbon gum stock and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as internal mixers, (e.g., Banbury mixers), roll mills, or any other convenient mixing device. The temperature of the mixture during the mixing process typically should not rise above about 120xc2x0 C. During mixing, it is preferable to distribute the components and adjuvants uniformly throughout the gum for effective cure.
The mixture is then processed and shaped, such as by extrusion (e.g., into the shape of a tube or a hose lining) or by molding (e.g., in the form of an O-ring). The shaped article can then be heated to cure the gum composition and form a cured article.
Molding or press curing of the compounded mixture usually is conducted at a temperature sufficient to cure the mixture in a desired time duration under a suitable pressure. Generally, this is between about 95xc2x0 C. and about 230xc2x0 C., preferably between about 150xc2x0 C. and about 205xc2x0 C., for a period of from about 1 minute to 15 hours, typically from 5 minutes to 30 minutes. A pressure of between about 700 kPa and about 20,600 kPa is usually imposed on the compounded mixture in a mold. The molds first may be coated with a release agent and prebaked.
The molded mixture or press-cured article is then usually post-cured (e.g., in an oven) at a temperature and for a time sufficient to complete the curing, usually between about 150xc2x0 C. and about 300xc2x0 C., typically at about 230xc2x0 C., for a period of from about 2 hours to 50 hours or more, generally increasing with the cross-sectional thickness of the article. For thick sections, the temperature during the post cure is usually raised gradually from the lower limit of the range to the desired maximum temperature. The maximum temperature used is preferably about 300xc2x0 C., and this value is held for about 4 hours or more. This post-cure step generally completes the cross-linking and may also release residual volatiles from the cured compositions. One example of a suitable post-cure cycle involves exposing molded parts to heat under nitrogen for 16 hours at 230xc2x0 C. The parts are returned to ambient temperature such as by shutting off oven heat.
The fluoropolymor compositions are useful in production of articles such as O-rings, gaskets, tubing, and seals, Such articles are produced by known methods. One such method includes, for example, molding a compounded formulation of the fluoroelastomer composition with various additives under pressure, curing the article, and then subjecting it to a post-cure cycle. The fluoroelastomers of the invention, after curing, exhibit significantly reduced compression set as compared to known elastomers. This improvement is most pronounced with TFE-P elastomers, while other polymers such as 70 wt % fluorine terpolymers or 66% fluorine copolymer elastomers may not exhibit this benefit to the extent seen in TFE-P elastomers.
Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.